ble, they
can never give us the idea of extension.

We may illustrate this by considering the sense of feeling, and the
imaginary distance or interval interposed betwixt tangible or solid
objects. I suppose two cases, viz. that of a man supported in the air,
and moving his limbs to and fro, without meeting any thing tangible; and
that of a man, who feeling something tangible, leaves it, and after a
motion, of which he is sensible, perceives another tangible object; and
I then ask, wherein consists the difference betwixt these two cases?
No one will make any scruple to affirm, that it consists meerly in the
perceiving those objects, and that the sensation, which arises from the
motion, is in both cases the same: And as that sensation is not capable
of conveying to us an idea of extension, when unaccompanyed with some
other perception, it can no more give us that idea, when mixed with
the impressions of tangible objects; since that mixture produces no
alteration upon it.

But though motion and darkness, either alone, or attended with tangible
and visible objects, convey no idea of a vacuum or extension without
matter, yet they are the causes why we falsly imagine we can form such
an idea. For there is a close relation betwixt that motion and darkness,
and a real extension, or composition of visible and tangible objects.

First, We may observe, that two visible objects appearing in the midst
of utter darkness, affect the senses in the same manner, and form the
same angle by the rays, which flow from them, and meet in the eye, as if
the distance betwixt them were find with visible objects, that give us
a true idea of extension. The sensation of motion is likewise the same,
when there is nothing tangible interposed betwixt two bodies, as when
we feel a compounded body, whose different parts are placed beyond each
other.

Secondly, We find by experience, that two bodies, which are so placed
as to affect the senses in the same manner with two others, that have a
certain extent of visible objects interposed betwixt them, are
capable of receiving the same extent, without any sensible impulse or
penetration, and without any change on that angle, under which they
appear to the senses. In like manner, where there is one object, which
we cannot feel after another without an interval, and the perceiving
of that sensation we call motion in our hand or organ of sensation;
experience shews us, that it is possible the same object may